Integrated intermodulation detection sub-system for telecommunications systems

ABSTRACT

Certain aspects are directed to an intermodulation detection sub-system. The intermodulation detection sub-system includes a test signal generation module, at least one intermodulation detection device, and a controller. The test signal generation module is integrated into a unit of a telecommunications system. The test signal generation module is configured to provide a test signal to a remote antenna unit of the telecommunications system. The intermodulation detection device is integrated into the telecommunications system. The intermodulation detection device is configured to detect intermodulation products generated by mixing a first signal component and a second signal component of the test signal. The controller is integrated into the unit. The controller is configured to control the test signal generation module and the at least one intermodulation detection device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase patent application of International Patent Application No. PCT/US2012/055793 filed Sep. 17, 2012 and titled “Integrated Intermodulation Detection Sub-System for Telecommunications Systems,” which claims priority to U.S. Provisional Application Ser. No. 61/535,520 filed Sep. 16, 2011 and titled “Integrated Passive Intermodulation Measurement Sub-System for Telecommunications Systems” and U.S. Provisional Application Ser. No. 61/637,382 filed Apr. 24, 2012 and titled “Intermodulation Detection Sub-System for Distributed Antenna System”, the contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates generally to signal processing in telecommunications systems and more particularly relates to detecting intermodulation products in telecommunications systems, such as distributed antenna systems.

BACKGROUND

A telecommunications system, such as a distributed antenna system (“DAS”), can provide signal coverage to coverage zones in which wireless devices are located. Signals communicated via a DAS or other telecommunications system can include signal components, such as passive intermodulation (“PIM”) products or other intermodulation products, that distort the signals being communicated. Intermodulation products can result from the undesired mixing of signals in a telecommunications system. Intermodulation products can be generated from two tones at different frequencies (e.g., f₁ and f₂) encountering a non-linear discontinuity or other non-linear interface in a telecommunications system. Examples of a non-linear discontinuity or other non-linear interfaces can include junctions of dissimilar metals in components of a signal path, such as the connections between an antenna and a transmitter. The intermodulation products can include signals having frequencies that are related to the frequencies of the original tones by the relationship

${f_{n} = {{\frac{n + 1}{2}f_{1}} - {\frac{n - 1}{2}f_{2}}}},$ where n=±3, ±5, ±7 . . . and is the order of the intermodulation product, f₁ is the frequency of the lower tone and f₂ is the frequency of the higher tone. For example, f₃=2×f₁−f₂ and is the frequency of a third order intermodulation product at a frequency below the frequencies of the two tones. Additionally, f⁻³=2×f₂−f₁ and is the frequency of a third order intermodulation product at a frequency above the frequencies of the two tones. The presence of these intermodulation products can indicate that intermodulation is present in the telecommunications subsystem.

A PIM measurement device that is not integral to or otherwise integrated with a telecommunications system can be used to measure PIM products of individual system components before installing each component in the system. This solution does not, however, allow for detecting and measuring the PIM products of an operational telecommunications system once the individual system components have been installed.

Accordingly, it is desirable to detect and measure intermodulation products in a telecommunications system subsequent to installation.

SUMMARY

In one aspect, an intermodulation detection sub-system is provided. The intermodulation detection sub-system includes a test signal generation module, at least one intermodulation detection device, and a controller. The test signal generation module is integrated into a unit of a telecommunications system. The test signal generation module is configured to provide a test signal to a remote antenna unit of the telecommunications system. The intermodulation detection device is integrated into the telecommunications system. The intermodulation detection device is configured to detect intermodulation products generated by mixing a first signal component and a second signal component of the test signal. The controller is integrated into the unit. The controller is configured to control the test signal generation module and the at least one intermodulation detection device.

In another aspect, an intermodulation detection sub-system is provided. The intermodulation detection sub-system includes a test signal generation module, an power measurement device, and a controller. The test signal generation module is integrated into a unit of a telecommunications system. The test signal generation module is configured to provide a test signal to a remote antenna unit of the telecommunications system. The power measurement device is integrated into the remote antenna unit. The power measurement device is configured to detect intermodulation products generated by mixing a first signal component and a second signal component of the test signal. The controller is integrated into the unit. The controller is configured to control the test signal generation module and the at least one power measurement device.

In another aspect, an intermodulation detection sub-system is provided. The intermodulation detection sub-system includes a test signal generation module that includes a first buffer, a second buffer, and a processor. The first buffer is communicatively coupled to a downlink path of a telecommunications system. The test signal generation module is configured to provide a test signal to the downlink path. The second buffer is communicatively coupled to an uplink path of the telecommunications system. The second buffer is configured to generate an uplink digital data set from an uplink signal traversing the uplink path. The processor is communicatively coupled to the first buffer and the second buffer. The processor is configured to detect an intermodulation product in an uplink signal traversing the uplink path by correlating a model intermodulation product and the uplink digital data set from the second buffer. The model intermodulation product is generated based on a test digital data set from the first buffer that represents the test signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a telecommunications system having an integrated intermodulation detection sub-system according to one aspect.

FIG. 2 is a schematic view of an example telecommunications system in which an intermodulation detection sub-system can be integrated according to one aspect.

FIG. 3 is a schematic view of a base station router with a test signal generator of an intermodulation detection sub-system according to one aspect.

FIG. 4 is a block diagram of a remote antenna unit having a passive intermodulation (“PIM”) measurement device of an intermodulation detection sub-system according to one aspect.

FIG. 5 is a schematic view of a PIM measurement device of an intermodulation detection sub-system according to one aspect.

FIG. 6 is a block diagram of an intermodulation detection sub-system configured to perform active detection of intermodulation products according to one aspect.

FIG. 7 is a partial schematic diagram of an intermodulation detection sub-system configured to perform active detection of intermodulation products according to one aspect.

FIG. 8 is a block diagram of an intermodulation detection sub-system configured to perform passive detection of intermodulation products according to one aspect.

FIG. 9 is a partial schematic diagram of a data capture module of an intermodulation detection sub-system configured to perform passive detection of intermodulation products according to one aspect.

FIG. 10 is a block diagram of a data processing module of an intermodulation detection sub-system configured to perform passive detection of intermodulation products according to one aspect.

DETAILED DESCRIPTION

Certain aspects and examples are directed to an intermodulation detection sub-system that can be disposed in a telecommunications system, such as a distributed antenna system (“DAS”). Downlink signals communicated via the telecommunications system can induce passive intermodulation (“PIM”) products at frequencies in the uplink frequency band that can be recovered by the remote antenna unit. The PIM products recovered by a remote antenna unit can distort uplink signals from wireless devices. The intermodulation detection sub-system can detect and/or measure PIM products and other intermodulation products. Integrating a PIM measurement sub-system into a telecommunications system can include disposing the components of the PIM measurement sub-system within one or more components of the telecommunications system. Using an integrated intermodulation detection sub-system can obviate the need for a separate, non-integral PIM measurement device.

In some aspects, the integrated sub-system can actively detect PIM products using test signals. The PIM measurement sub-system can include a controller, a test signal generator, and a PIM measurement device for actively detecting PIM products. The test signal generator can generate a test signal for measuring PIM products and provide the test signal to a downlink path of the telecommunications system. The test signal can include signal components at frequencies selected by the controller such that a linear combination of the first and second frequencies, such as (but not limited to) the sum or difference of the frequencies, is a frequency in the uplink frequency band. If the test signal encounters a non-linear interface, PIM products can be generated from the signal components of the test signal mixing at the non-linear interface. PIM products generated in the downlink path can be recovered by a receiving antenna and provided to the uplink path.

In some aspects, a PIM measurement device, such as a power meter, included in one or more components of the telecommunication system can detect and measure the PIM products in the uplink path. The power meter can measure PIM products in signals received via test probes coupled to uplink paths. Examples of a power meter can include a logarithmic (“LOG”) detector or a root mean square (“RMS”) detector. In other aspects, PIM products can be detected by correlating a model intermodulation product and an uplink digital data set that digitally represents an uplink signal including the intermodulation products. A model intermodulation product can include a data set that is mathematically derived from a test digital data set that digitally represents the test signal. For example, a model intermodulation product may be a digital representation of a linear combination of the first and second frequencies of a test signal. A PIM measurement device can include a capture buffer configured to store the uplink digital data set. The capture buffer can be synchronized with a signal generation module generating the test digital data set. The controller can mathematically correlate model intermodulation product with the uplink digital data set. Mathematically correlating the model intermodulation product with the uplink digital data set can indicate the presence of the intermodulation product in the uplink signal.

In additional or alternative aspects, the intermodulation detection sub-system can perform passive detection of PIM products. The intermodulation detection sub-system can include a downlink capture module coupled to the downlink path and an uplink capture module coupled to the uplink path. The downlink capture module can receive a downlink signal from the downlink path and convert the downlink signal to a downlink digital data set representing the downlink signal. The uplink capture module can receive a uplink signal from the uplink path and convert the uplink signal to an uplink digital data set representing the uplink signal. The intermodulation-modeling module can apply a non-linear function, which models the generation of an intermodulation product from the downlink signal, to the downlink digital data set to generate a model intermodulation product. The processor can cross correlate the model intermodulation product with the uplink digital data set to determine that a mathematical correlation exists between the model intermodulation product and the uplink digital data set, thereby indicating the presence and signal power of an intermodulation product in the uplink path.

Detailed descriptions of certain examples are discussed below. These illustrative examples are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts. The following sections describe various additional aspects and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative examples but, like the illustrative examples, should not be used to limit the present invention.

FIG. 1 depicts an intermodulation detection sub-system 103 integrated into a telecommunications system 100. Integrating the intermodulation detection sub-system 103 in the telecommunications system 100 can include disposing some or all components of the intermodulation detection sub-system 103 enclosed within one or more communication devices of the telecommunication system 100. The telecommunications system 100 in FIG. 1 also includes a downlink path 104 and an uplink path 106. The downlink path 104 can be a signal path including one or more devices for communicating downlink signals received from a base station to wireless devices serviced by the telecommunication system 100. Downlink signals are signals at frequencies in a downlink frequency band provided from one or more base stations 102 a-n to a remote antenna unit for radiation to wireless devices. The uplink path 106 can be a signal path including one or more devices for communicating uplink signals received from wireless devices to one or more base stations 102 a-n. Uplink signals can include signals received from wireless devices in the coverage zones serviced by remote antenna units of the telecommunication system 100. The intermodulation detection sub-system 103 can detect and measure PIM products in signals traversing the downlink path 104 and/or the uplink path 106.

FIG. 2 depicts an example telecommunications system 100 having a base station router 202 in communication with base stations 102 a-n and a distribution unit 204. The telecommunications system 100 can also include master units 206 a, 206 b in communication with the distribution unit 204 and the remote antenna units 208 a-f. The telecommunications system 100 can be positioned in an area to extend wireless communication coverage or to facilitate other telecommunication functions.

In the direction of a downlink path 104, the telecommunications system 100 can receive downlink signals from the base stations 102 a-n via a wired or wireless communication medium. Downlink signals can be received by the base station router 202. The base station router 202 can provide the downlink signals to the master units 206 a, 206 b via the distribution unit 204. The master units 206 a, 206 b can communicate with the distribution unit 204 via any communication medium capable of carrying signals between the distribution unit 204 and the master units 206 a, 206 b. Examples of a suitable communication medium include copper wire (such as a coaxial cable), optical fiber, and microwave or optical link. The link can transport the signals in analog or in digitized form. In some aspects, the master units can communicate directly with the base station router 202.

The master units 206 a, 206 b can provide downlink signals to the remote antenna units 208 a-f. The remote antenna units 208 a-f can communicate with the master units 206 a, 206 b via any communication medium capable of carrying signals between the master units 206 a, 206 b and the remote antenna units 208 a-f. Examples of a suitable communication medium include copper wire (such as a coaxial cable), optical fiber, and microwave or optical link. The link can transport the signals in analog or in digitized form. The remote antenna units 208 a-f can radiate the signals of the sector(s) distributed to the physical area.

In the direction of an uplink path 106, the base station router 202 can receive uplink signals from remote antenna units 208 a-f via the master units 206 a, 206 b and the distribution unit 204.

An intermodulation detection sub-system 103 can be integrated into the telecommunications system 100 depicted in FIG. 2. One or more component of the intermodulation detection sub-system 103 can be disposed in one or more of the components of the telecommunications system 100.

Intermodulation Detection Sub-System in Base Station Router

FIG. 3 depicts an example of a base station router 202 in which an intermodulation detection sub-system 103 is integrated. The intermodulation detection sub-system 103 can include a controller 302, a test signal generator 304, a path switch 310, test switches 312, terminating switches 314, protection switches 316, detector switches 318 a, 318 b, power meters 320 a, 320 b, and test probes 322 a-d. Although the base station router 202 is depicted as having two downlink paths 104 a, 104 b and two uplink paths 106 a, 106 b, the base station router 202 can include any number of uplink and downlink paths, including one of each.

The controller 302 can configure the components of the intermodulation detection sub-system 103. Configuring the components of the intermodulation detection sub-system 103 can include modifying operation of a component within the telecommunication system 100 or otherwise causing a change in the state of a component. The controller 302 can configure the components by, for example, generating control signals and transmitting control signals to the respective components. An example of a controller 302 is a Peripheral Interface Controller (“PIC”). The controller 302 can communicate with the components of the intermodulation detection sub-system 103 integrated into the base station router 202 via, for example, a printed circuit board. The controller 302 can communicate with components of the intermodulation detection sub-system 103 disposed elsewhere in the telecommunications system 100 (e.g., in the master units, the remote antenna units, etc.) using control signals communicated via the downlink paths 104 a, 104 b and uplink paths 106 a, 106 b.

In additional or alternative aspects, the controller 302 can communicate with components of the intermodulation detection sub-system 103 disposed elsewhere in the telecommunications system 100 using control signals communicated via a control path. The control path can be any communication medium suitable for wired or wireless communication between components of the intermodulation detection sub-system 103. Examples of a suitable communication medium include copper wire (such as a coaxial cable), optical fiber, and microwave or optical link.

The test signal generator 304 can include signal sources 306 a, 306 b and a combiner 308. A signal source can be, for example, an analog signal generator capable of producing continuous wave tones. The combiner 308 can combine signals from each of the signal sources 306 a, 306 b to output a combined test signal. In some aspects, combining the signals can include summing or adding signals. The controller 302 can be used to configure the signal sources 306 a, 306 b to select the frequencies of the signals from the signal sources 306 a, 306 b. The frequencies of the two test signals can be selected such that a linear combination of the test signal frequencies is a frequency in the uplink frequency band. In some aspects, the test signal generator 304 may be integrated in a PIM measurement device, which may be integrated in a remote antenna unit. In additional or alternative aspects, the test signal generator 304 may use n-tone stimulation (e.g. triple beat measurement) or use two tones in different downlink paths.

The test signal generator 304 can provide a combined test signal to the downlink paths 104 a, 104 b via the path switch 310 and the test switches 312. The path switch 310 can be used to select either of downlink paths 104 a, 104 b to receive the combined test signal. The test switches 312 can be used to couple the path switch 310 to either of the downlink paths 104 a, 104 b.

The PIM products for the telecommunications system 100 can be detected and measured by configuring the telecommunications system 100 for testing. In test mode, the controller 302 can configure the intermodulation detection sub-system 103 to couple one or both of the downlink paths 104 a, 104 b to the path switch 310 via one or both of the test switches 312. The controller 302 can configure the intermodulation detection sub-system 103 to terminate the input from the base stations to the downlink paths 104 a, 104 b. Terminating the input from the base stations to the downlink paths 104 a, 104 b can include coupling the base stations via the terminating switches 314 to terminating resistive loads, each depicted in FIG. 3 as a resistor coupled to ground. The controller 302 can also configure the intermodulation detection sub-system 103 to ground the output of one or both of the uplink paths 106 a, 106 b by coupling the uplink paths 106 a, 106 b to terminating resistive loads via the protection switches 316.

The power meter 320 a can measure the power of signals received by test probes 322 a, 322 b in the downlink paths 104 a, 104 b, respectively. The power meter 320 a can measure the power of the combined test signal provided by the test signal generator 304. The power meter 320 a can be, for example, a logarithmic (“LOG”) detector or a root mean square (“RMS”) detector. The controller 302 can configure the detector switch 318 a to couple the power meter 320 a to either of the test probes 322 a, 322 b.

If the combined test signal encounters a non-linear interface in the downlink path or the uplink path, PIM products can be generated from the mixing of the signal components of the test signal at the non-linear interface. Such undesirable mixing can occur in the downlink path or the uplink path. The PIM products can be transmitted and recovered by a remote antenna unit and provided to the uplink paths 106 a, 106 b.

The power meter 320 b can measure, at the base station router 202, PIM products in signals received by the test probes 322 c, 322 d in the uplink paths 106 a, 106 b, respectively. The power meter 320 b can be, for example, a logarithmic (“LOG”) detector or a root mean square (“RMS”) detector. The controller 302 can configure the detector switch 318 b to couple the power meter 320 b to either of the test probes 322 c, 322 d.

In some aspects, either or both of the power meters 320 a, 320 b, as well as the associated detector switches 318 a, 318 b and test probes 322 c-d, can be omitted.

FIG. 3 also depicts the base station router 202 as including attenuators 326 a-d and attenuator path switches 328 in the downlink paths 104 a, 104 b and attenuators 326 e-h and attenuator path switches 328 in the uplink paths 106 a, 106 b. Attenuators 326 c, 326 d can equalize the power of downlink signals from different base stations prior to combining the downlink signals at the distribution unit 204. Attenuators 326 g, 326 h can attenuate the power of uplink signals after the uplink signals have been de-multiplexed by the distribution unit 204 such that the uplink signals are at the signal strength required by the base stations receiving the uplink signals

In a test mode, the controller 302 can configure the telecommunications system 100 such that downlink signals from the base stations are routed to the attenuators 326 a, 326 b via the attenuator path switches 328. The attenuators 326 a, 326 b can attenuate the downlink signals. The attenuated downlink signals can be routed to the terminating resistive loads via the attenuator path switches 328 and the terminating switches 314.

Intermodulation Detection Sub-System in Remote Antenna Unit

In additional or alternative aspects, part of the intermodulation detection sub-system 103 can be integrated into other components of the telecommunications system 100. For example, FIG. 4 depicts an example of a remote antenna unit 208 having a detection and measurement components of the intermodulation detection sub-system 103. The remote antenna unit 208 can include the power amplifiers 403 a, 403 b, the isolation sub-systems 404 a, 404 b, the low noise amplifiers 405 a, 405 b, and an antenna 410. The measurement components of the intermodulation detection sub-system 103 can include a sub-controller 401, a PIM measurement device 402, test switches 406 a, 406 b, path switch 408, and test probes 412 a-d.

The sub-controller 401 can configure the components of the intermodulation detection sub-system 103 integrated into the remote antenna unit 208. An example of a sub-controller 401 is a PIC. The sub-controller 401 can communicate with the components of the intermodulation detection sub-system 103 integrated into the remote antenna unit 208 via, for example, a printed circuit board. The sub-controller 401 can communicate with the controller 302 using control signals communicated via the downlink paths 104 a, 104 b and uplink paths 106 a, 106 b.

The remote antenna unit 208 can receive downlink signals via the downlink paths 104 a, 104 b and provide uplink signals via the uplink paths 106 a, 106 b. The isolation sub-system 404 a can isolate downlink signals traversing the downlink path 104 a, transmitted via the antenna 410 from uplink signals traversing the uplink path 106 a and recovered via the antenna 410. The isolation sub-system 404 b can isolate downlink signals traversing the downlink path 104 b from uplink signals traversing the uplink path 106 b. The isolation sub-systems 404 a, 404 b can each include, for example, a duplexer. The isolation sub-systems 404 a, 404 b can include one or more non-linear interfaces that can generate PIM products.

The PIM measurement device 402 can be coupled to test probes 412 a-d via the test switches 406 a, 406 b and the path switch 408. The test probes 412 a, 412 b can receive a signal to be measured from the downlink path 104 a or the uplink path 106 a, respectively. The test probes 412 c, 412 d can receive a signal to be measured from the downlink path 104 b and the uplink path 106 b, respectively. The PIM measurement device 402 can receive the signal to be measured via the path switch 408.

In a test mode, the controller 302 can configure the intermodulation detection sub-system 103 via a sub controller to couple the path switch 408 to test probes 412 a, 412 b or to test probes 412 c, 412 d. The controller 302 can configure the intermodulation detection sub-system 103 via the sub controller to couple the test switch 406 a to either of the test probes 412 a, 412 b. The controller 302 can configure the intermodulation detection sub-system 103 to couple the test switch 406 a to either of the test probes 412 c, 412 d.

The PIM measurement device 402 can detect and measure PIM products in signals traversing the downlink paths 104 a, 104 b by measuring a signal received via the test probes 412 a, 412 c respectively. The PIM products in the downlink paths 104 a, 104 b can be generated by components in the downlink paths 104 a, 104 b prior to the isolation sub-systems 404 a, 404 b. Components in the downlink paths 104 a, 104 b that can generate PIM products can include, for example, the power amplifiers 403 a, 403 b.

When the controller 302 configures the intermodulation detection sub-system 103 to measure a signal received via the test probes 412 b, 412 d, the PIM measurement device 402 can detect and measure PIM products in signals traversing the uplink paths 106 a, 106 b after low noise amplifiers 405 a, 405 b. The PIM products in the uplink paths 106 a, 106 b can also include PIM products from the downlink paths 104 a, 104 b.

In some aspects, the PIM measurement device 402 can be used as a spectrum analyzer to measure signals prior to isolation sub-system 404 a using test probes 412 a-b and subsequent to isolation sub-system 404 a using a test probe (not shown). Furthermore, test probes 412 a, 412 c can be used to measure intermodulation and/or internal PIM products generated from the system. Because of downlink and uplink isolation, internal intermodulation can be determined using test probes 412 b, 412 d and PIM measurement device 402. Test probes 412 b, 412 d can be used to measured a composite (i.e. external and internal) PIM. Using the internal intermodulation determined using test probes 412 b, 412 d, exclusively external PIM can be measured. In some aspects, the PIM measurement device 402 can be used as a pilot generator to measure other system characteristics, such as uplink gain. In other aspects, the PIM measurement device can be configured to digitally sample and process a measurement signal.

Although the intermodulation detection sub-system 103 is depicted in FIGS. 3 and 4 as including a test signal generator 304 integrated into the base station router 202 and a PIM measurement device 402 integrated into a remote antenna unit 208, these PIM measurement sub-system components can be integrated in other devices of the telecommunications system. For example, in some aspects, both the test signal generator 304 and the PIM measurement device 402 can be integrated into a remote antenna unit 208.

The remote antenna unit 208 can also include a frequency scanner 414 for distinguishing extraneous signals from PIM products. Extraneous signals can include signals from other wireless devices in the coverage area of the remote antenna unit 208. The frequency scanner 414 can include an antenna and a spectrum analyzer. In some aspects, the frequency scanner 414 can communicate with the controller 302 using control signals transmitted via one or more of the uplink and downlink paths. In other aspects, the frequency scanner can communicate with the controller 302 using control signals transmitted via a control path. The controller 302 can configure the frequency scanner 414 to detect extraneous signals using an antenna separate from the antenna 410. The frequency scanner to 314 can determine the frequencies of any detected extraneous signals.

The controller 302 can detect the presence of any extraneous signals in the coverage area of the remote antenna unit 208 at the same frequencies as any PIM products generated by mixing of the signal components of the test signal. By determining if any extraneous signals have the same frequencies as the PIM products, the controller 302 can distinguish the PIM products in the uplink paths 106 a, 106 b from any extraneous signals. In some aspects, the intermodulation detection sub-system 103 can identify the frequencies of extraneous signals prior to generating the test signal. The controller 302 can select test signal frequencies such that any PIM products will be at frequencies in the uplink frequency band while avoiding frequencies of any extraneous signals in the coverage area of the remote antenna unit 208.

Although the PIM measurement device 402 is depicted as including test probes for two downlink paths and two uplink paths, a PIM measurement device can include test probes for any number of downlink paths and uplink paths. Furthermore, an intermodulation detection sub-system 103, according to some aspects, can be implemented without a frequency scanner.

FIG. 5 schematically depicts an example of the PIM measurement device 402 of the intermodulation detection sub-system 103. The PIM measurement device 402 can include a local oscillator 502, a mixer 504, a filter switch 506, filters 508 a-d, and a power meter 510.

The local oscillator 502 and the mixer 504 can receive a signal from a downlink path or an uplink path via the filter switch 506. The local oscillator 502 and the mixer 504 can down-convert the signal to an intermediate frequency.

The filters 508 a-d can be included in a switched filter bank. Filters 508 a-d can be used to measure characteristics using different bandwidth resolutions. Each of the filters 508 a-d in the switched filter bank can pass a different frequency bands at the filtering stage. One of the filters 508 a-d can receive the signal via the filter switch 506. The filters 508 a-d can filter the signal to isolate any PIM products and minimize noise. The filters 508 a-d can cancel or attenuate any signal components of the received signal other than PIM products. Each of the filters 508 a-d can pass a different frequency band. One of the filters 508 a-d can be selected using the filter switch 506. A filter associated with a given frequency band to be passed can be selected, for example, such that the frequencies corresponding to extraneous signals can be rejected or attenuated. The frequency band to be passed can correspond to the frequency of PIM products included in the signal.

The power meter 510 can receive the signal from one of the filters 508 a-d. The power meter can be, for example, a LOG detector or an RMS detector. If the power meter 510 detects a PIM product, the PIM measurement device 402 can output an alarm message identifying the presence of the PIM product to other components of the telecommunication system 100, such as (but not limited to) the base station router 202. The PIM measurement device 402 can provide the alarm message to other components of the telecommunication system 100 via one of the uplink paths 106 a, 106 b.

In some aspects, the intermodulation detection sub-system 103 can be configured to periodically generate test signals and measure the resulting PIM products. The controller 302 can synchronize the test signal generator 304 and the local oscillator 502 such that the necessary resolution bandwidth for the power meter 510 is minimized. By periodically generating test signals and measuring the resulting PIM products, the intermodulation detection sub-system 103 can distinguish PIM products from noise in the uplink paths 106 a, 106 b. In other aspects, the test signal generator 304 can generate test signals of sufficient power that any resulting PIM products are at a signal power exceeding the noise floor of the uplink paths 106 a, 106 b.

Active Detection of Intermodulation Products

In additional or alternative aspects, the intermodulation detection sub-system 103′ can perform active detection of intermodulation products. For example, FIG. 6 is a block diagram depicting an aspect of an intermodulation detection sub-system 103′ configured to perform active detection of intermodulation products. The intermodulation detection sub-system 103′ can include a processor 602, a test signal generation module 604, and a data capture module 606.

The intermodulation detection sub-system 103′ can detect intermodulation products, such as passive intermodulation products generated by a non-linear impedance source 608. Examples of a non-linear impedance source 608 can include junctions of dissimilar metals in components of a signal path, such as the connections between an antenna and a transmitter. A telecommunications system 100 can include multiple non-linear impedance sources 608, such as the remote antenna units 208 a-f.

The processor 602 can be communicatively coupled to the test signal generation module 604 and the data capture module 606. The processor 602 can process data from the test signal generation module 604 and the data capture module 606 to detect intermodulation products in the uplink path 106. Examples of the processor 602 include a microprocessor, a peripheral interface controller (“PIC”), an application-specific integrated circuit (“ASIC”), a field-programmable gate array (“FPGA”), or other suitable processor. The processor 602 may include one processor or any number of processors.

The processor 602 can configure the test signal generation module 604 to generate an analog test signal from a test digital data set. An example of a test signal generation module 604 can include a periodic playback buffer, a digital-to-analog converter, up-conversion circuitry, and a power amplifier. The test signal generation module 604 can be communicatively coupled to the downlink path 104. The test digital data set can represent a digital signal having two test frequencies. The two test frequencies can be selected by the processor 602 such that a linear combination of the test frequencies is a frequency in the uplink frequency band. The test signal generation module 604 can convert the test digital data set to an analog test signal having frequency components for each of the test signal frequencies.

The test signal generation module 604 can provide the analog test signal to the downlink path 104. If the analog test signal encounters a non-linear impedance source 608, PIM products or other intermodulation products can be generated from the signal components of the analog test signal mixing at the non-linear interface of the non-linear impedance source 608. The PIM products can have a frequency within the uplink frequency band. PIM products generated in the downlink path can be provided to the uplink path via reflection by the non-linear impedance source 608 or via recovery by a receiving antenna, such as a remote antenna unit.

The data capture module 606 can be communicatively coupled to the uplink path 106. An example of a data capture module 606 can include a low-noise amplifier, down-conversion circuitry, an analog-to-digital converter, and an averaging capture buffer. Analog signals traversing the uplink path 106 can be received by the data capture module 606. The data capture module 606 can convert signals traversing the uplink path 106 to an uplink digital data set representing a digital uplink signal.

The test signal generation module 604 and the data capture module 606 can be synchronized. Synchronizing the test signal generation module 604 and the data capture module 606 can allow the processor 602 to identify a signal component of a signal traversing the uplink path 106 that is an intermodulation product. The processor 602 can generate a model intermodulation product from the test digital data set that can be mathematically correlated with the intermodulation product in the uplink path 106. The processor 602 can determine that an increase in signal power of an uplink signal detected by the data capture module 606 is mathematically correlated with the model intermodulation product generated using a test signal generated by the test signal generation module 604. An increase in signal power of an signal detected by the data capture module 606 that is mathematically correlated with the model intermodulation product can indicate the presence and signal power of an intermodulation product in the uplink path 106.

Although the intermodulation detection sub-system 103′ is depicted in FIG. 2 as being disposed in the master unit 206, other configurations are possible. The intermodulation detection sub-system 103′ can be disposed in or integrated with any device in a telecommunications system 100. In some aspects, an intermodulation detection sub-system 103′ can be disposed in each of the remote antenna units 208 a-f. In other aspects, an intermodulation detection sub-system 103′ can be disposed in the master unit 206 and each of the remote antenna units 208 a-f. In other aspects, an intermodulation detection sub-system 103′ can be disposed in a base station router 202.

FIG. 7 is a partial schematic diagram of an intermodulation detection sub-system 103′ configured to perform active detection of intermodulation products according to one aspect. The test signal generation module 604 can generate an analog signal, such as an RF signal, which can include two test signal components. The data capture module 606 can detect the presence of an intermodulation product.

An example of a test signal generation module 604 is a playback buffer, such as a periodic playback buffer. In some aspects, the test signal generation module 604 can be loaded with an L-point data set containing two complex tones, such as

${{x\lbrack n\rbrack} = {{a_{1}{\mathbb{e}}^{{\frac{2\;\pi}{L}k_{1}n} + \theta_{2}}} + {a_{2}{\mathbb{e}}^{{\frac{2\;\pi}{L}k_{2}n} + \theta_{2}}}}},$ where a_(i) is the amplitude, θ_(i) is the phase, and k_(i) is the frequency index of the tones. Both of the complex tones can have a periodicity of L samples. The subscripts on the amplitudes, phases, and delays can include two indices. The first index can identify the tone. The second index can identify delineate the stage of the system. For example, a_(i,2) is the amplitude of a tone i in the second stage.

The test signal generation module 604 can provide the data set x[n] to an up-conversion module 702. An example up-conversion module 702 depicted in FIG. 7 is a complex modulator. The up-conversion module 702 can include a digital-to-analog converter 705, filters 706 a, 706 b, a mixer 708, and a local oscillator 710. The up-conversion module 702 can up-convert complex tones to an RF frequency. The complex tones can be amplified by a power amplifier 712. The output of the power amplifier 712 can be an analog signal represented by the function x(t)=a _(1,2) cos(2πf ₂ t+θ _(1,2))+a _(2,2) cos(2πf ₂+θ_(2,2))+d _(t,2) cos(2πf _(n) t+φ _(t,2)). Each of the frequencies f_(i) can be represented by the function

${f_{i} = {f_{tx} + {\frac{f_{s}}{L}k_{i}}}},$ where f_(s) is the sample rate and f_(tx) is the local oscillator frequency. The analog signal can be provided to a transmission line 716 via the downlink side of an isolation sub-system 714, such as (but not limited to) a duplexer.

In addition to the two tones, the analog signal outputted from the power amplifier 712 can include intermodulation products generated by the up-conversion module 702 and the power amplifier 712. The frequencies f_(n) of the intermodulation products can be represented by the function

$f_{n} = {{\frac{n + 1}{2}f_{1}} + {\frac{n - 1}{2}f_{2}}}$ with a phase of φ_(t,2).

The frequencies of the complex tones, f₁ and f₂, can be selected such that the complex tones can be provided to the transmission line 716 via the downlink side of the isolation sub-system 714 without being significantly attenuated. The desired intermodulation product f_(n) to be measured is significantly attenuated by downlink side of the isolation sub-system 714. The desired intermodulation product f_(n) to be measured is not significantly attenuated by the uplink side of the isolation sub-system 714.

A signal generated from the complex tones can be provided to the transmission line 716 via the downlink side of the isolation sub-system 714. The signal at the output of the isolation sub-system 714 can be represented by the function x _(f)(t)=a _(1,3) cos(2πf ₁ t+θ _(1,3))+a _(2,3) cos(2πf ₂ t+θ _(2,3)).

The complex signal can traverse the transmission line 716 from the output of the isolation sub-system 714 to one or more points of non-linear discontinuities. The one or more points of non-linear discontinuities are represented in FIG. 7 as non-linear impedance sources 608 a-c. A nonlinear discontinuity physically located at points pt_(i) at a distance of d_(i) meters from the isolation sub-system 714 can delay the signal traversing the transmission line, as represented by the function x _(i)(t)=a _(i)(a _(1,3) cos(2πf ₁(t−τ _(i))+θ_(1,3))+a _(2,3) cos(2πf ₂(t−τ _(i))+θ_(2,3))), where the signal has been delayed by

$\tau_{1} = \frac{d_{i}}{\left( {{vf} \times c} \right)}$ for a velocity factor vf and the speed of light c and attenuated by a factor of α_(i) caused by losses in the transmission line.

At points pt_(i) having nonlinear discontinuities, the complex signal can interact with the nonlinear discontinuities to generate intermodulation products represented by the function y_(i)(t)=b_(i,4) cos(2πf_(n)(t−τ_(i))+φ_(i,4)), where b_(i, 4) is the amplitude of the associated n^(th) order intermodulation product. The intermodulation products can traverse the transmission line 716 in a direction away from the isolation sub-system 714 and in a direction toward the isolation sub-system 714. Intermodulation products traversing the transmission line 716 in a direction away from the isolation sub-system 714 can be sufficiently small that other signals communicated via the telecommunications system 100 are not distorted.

Intermodulation products traversing the transmission line 716 in a direction toward the isolation sub-system 714 can combine with additional noise received by the telecommunications system 100. The intermodulation products combined with noise can be represented by the function y _(r)(t)=Σ_(i=1) ^(p) b _(i,3) cos(2πf _(n)(t−2τ_(i))+φ_(i,3))+e ₃(t), where e₃(t) is the noise.

The intermodulation products can be provided to the uplink path 106 via the isolation sub-system 714 and the low noise amplifier 718. The intermodulation products at the uplink output of the isolation sub-system 714 can be represented by the function

${y(t)} = {{\sum\limits_{i = 1}^{P}\;{b_{i,2}{\cos\left( {{2\;\pi\;{f_{n}\left( {t - {2\;\tau_{i}}} \right)}} + \phi_{i,2}} \right)}}} + {d_{r,2}{\cos\left( {{2\;\pi\; f_{n}t} + \varphi_{r,2}} \right)}} + {{e_{2}(t)}.}}$ where the terms b_(i,2) are associated with the nonlinear discontinuities in the RF subsystem, the term d_(r,2) is associated with the intermodulation generated in the power amplifier 712 that is not completely removed by the isolation sub-system 714, and e₃(t) is the received noise.

A down-conversion module 704 can down-convert the uplink signal including intermodulation products to an intermediate frequency (“IF”). The down-conversion module 704 can include a filter 720, a mixer 722, a local oscillator 724, a filter 726, and an analog-to-digital converter 728. The analog-to-digital converter 728 can sample the uplink signal including the intermodulation products. The output of the analog-to-digital converter 728 can be represented by the equation:

${y\lbrack n\rbrack} = {{\sum\limits_{i = 1}^{p}\;{b_{i}{\cos\left( {{2\;\pi\frac{f_{n} - f_{rx}}{f_{s}}n} - {4\;\pi\; f_{n}\tau_{i}} + \varphi_{i}} \right)}}} + {d_{r,2}{\cos\left( {{2\;\pi\frac{f_{n} - f_{rx}}{f_{s}}n} + {\varphi\; r}} \right)}} + {e_{1}\lbrack n\rbrack}}$

The tones in the above equation can have the same frequency and can have different amplitudes, delays, and phases.

The test signal generation module 604 and the data capture module 606 can have a common clock 730. The test signal generation module 604 and the data capture module 606 can be synchronized via the clock 730. The test signal generation module 604 and the data capture module 606 can include buffers having the same length. The buffer for the test signal generation module 604 that is outputting sample n can correspond to the buffer for the data capture module 606 that is recording sample n.

The test signal generation module 604 can generate a data set x[n] to drive the digital-to-analog converter 705. The data set x[n] can produce an analog signal to be provided to the downlink path. A processor 602, such as a microprocessor or digital signal processor, can load a playback buffer of the test signal generation module 604 with a length L data set x[0], x[1], . . . x[L−1]. The playback buffer can output data values starting with the value x[0] and continuing to the value x[L−1]. The playback buffer can repeat the data set such that a periodic sequence is generated with a period of L samples.

A capture buffer of the data capture module 606 can use synchronized averaging to capture the data from the analog-to-digital converter 728. The data stored in the capture buffer can be represented by the function

${y_{M}\lbrack n\rbrack} = {\sum\limits_{m = 0}^{M - 1}\;{y\left\lbrack {n + {mL}} \right\rbrack}}$ where M is the number of periods over which the average is taken. Using synchronized averaging for each period can allow the intermodulation detection sub-system 103′ to distinguish a signal component from a noise component. For example, a date set y[n]=x[n]+e[n] can be composed of a periodic signal component x[n] and a zero-mean ergodic noise component e[n] such that

${y_{M}\lbrack n\rbrack} = {\sum\limits_{m = 0}^{M - 1}{\left( {{x\left\lbrack {n + {mL}} \right\rbrack} + {e\left\lbrack {n + {mL}} \right\rbrack}} \right).}}$

The periodic nature of the data set x[n] can allow the data set y[n] to be represented by the function

${y_{M}\lbrack n\rbrack} = {{{Mx}\lbrack n\rbrack} + {\sum\limits_{m = 0}^{M - 1}{e\left\lbrack {n + {mL}} \right\rbrack}}}$ such that the signal component is increased by a factor of M and the noise component is not increased by a factor of M. For an ergodic noise component, the data set y_(M)[n] can be an estimator of the data set x[n], as demonstrated by the function

${\lim_{M->\infty}{\frac{1}{M}{y_{M}\lbrack n\rbrack}}} = {{{x\lbrack n\rbrack} + {E\left\{ {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{e\left\lbrack {n + {mL}} \right\rbrack}}} \right\}}} = {{{x\lbrack n\rbrack} + {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{E\left\{ {e\left\lbrack {n + {mL}} \right\rbrack} \right\}}}}} = {x\lbrack n\rbrack}}}$ where E{ } is an expectation operator. The data captured in the capture buffer can be made arbitrarily close to the periodic component of the signal by averaging a sufficient number of periods. Averaging the periods can allow the intermodulation detection sub-system 103′ to detect intermodulation products despite the presence of large amounts of noise in the telecommunications system 100.

To detect intermodulation products, such as passive intermodulation products, in the telecommunications system 100, the playback buffer of the test signal generation module 604 can be loaded with two tones. Any value can be used for the respective phases θ₁ and θ₂ of the tones. For a zero value of the phases θ₁ and θ₂, the tones can be loaded as a data set x[n] represented by the function

${x\lbrack n\rbrack} = {{a_{1}{\mathbb{e}}^{\frac{2\pi}{L}k_{1}n_{1}}} + {a_{2}{{\mathbb{e}}^{\frac{2\pi}{L}k_{2}n}.}}}$ The amplitudes a₁ and a₂ can be set such that the amplitudes of the two tones at the output of the isolation sub-system 714, a_(1,3) and a_(2,3), are equal.

The data set sampled by an averaging capture buffer of the data capture module 606 can be represented by the function

${y\lbrack n\rbrack} = {{\sum\limits_{i = 1}^{p}{b_{i}{\cos\begin{pmatrix} {{2\pi\;\frac{f_{n} - f_{rx}}{f_{s}}n} -} \\ {{4\pi\; f_{n}\tau_{i}} + \phi_{i}} \end{pmatrix}}}} + {d_{r,2}{\cos\left( {{2\pi\;\frac{f_{n} - f_{rx}}{f_{s}}n} + \varphi_{r}} \right)}} + {{e_{1}\lbrack n\rbrack}.}}$ The frequency of the tones can be represented by the function

${f_{n} - f_{rx}} = {\left( {f_{tx} - f_{rx}} \right) + {\frac{f_{s}}{L}{\left( {{\frac{n + 1}{2}k_{1}} - {\frac{n - 1}{2}k_{2}}} \right).}}}$

Setting the local oscillator 710, the local oscillator 724, and the analog-to-digital converter 728 to a common reference can allow the frequency difference between the local oscillator 710 and the local oscillator 724 to be represented by the function

${{f_{tx} - f_{rx}} = {\frac{f_{s}}{L}k_{c}}},$ for some integer k_(c). The tone frequency for this condition can be represented by the function

${f_{n} - f_{rx}} = {\frac{f_{s}}{L}{\left( {k_{c} + {\frac{n + 1}{2}k_{1}} - {\frac{n - 1}{2}k_{2}}} \right).}}$ The radian frequency of the tones can thus be represented by the function

$w_{n} = {{2\pi\frac{f_{n} - f_{rx}}{f_{s}}} = {\frac{2\pi}{L}{\left( {k_{c} + {\frac{n + 1}{2}k_{1}} - {\frac{n - 1}{2}k_{2}}} \right).}}}$ The digital data set captured by the capture buffer of the data capture module 606 can be represented by the function

${y\lbrack n\rbrack} = {{\sum\limits_{i = 1}^{p}{b_{i}{\cos\left( {{w_{n}n} - {4\pi\; f_{n}\tau_{i}} + \phi_{i}} \right)}}} + {d_{r,2}{\cos\left( {{w_{n}n} + \varphi_{r}} \right)}} + {{e_{1}\lbrack n\rbrack}.}}$

The digital data set can be a linear combination of tones having the same frequency such that individual amplitudes b_(i) are not determined. The sum of multiple tones having the same frequency can be represented by a single tone of the same frequency and scaled in amplitude and phase shifted. Thus, the test digital data set y[n] can be represented as a single tone in Gaussian noise. The maximum likelihood method for estimating the amplitude of a tone in Gaussian noise is to correlate the signal with a complex tone of the same frequency. The maximum likelihood method can thus be represented as

$\hat{b} = {\frac{1}{L}{\sum\limits_{n = 0}^{L - 1}{{y\lbrack n\rbrack}{\mathbb{e}}^{j\; w_{n}n}}}}$ where {circumflex over (b)} represents the estimated amplitude of the tone. For a sufficiently small noise value, the statistic {circumflex over (b)} can be represented by the function

$\hat{b} = {{\sum\limits_{n = 0}^{L - 1}{b_{i}{\mathbb{e}}^{j{({{4\pi\; f_{n}\tau_{i}} + \phi_{i}})}}}} + {d_{r}{{\mathbb{e}}^{{j\varphi}_{r}}.}}}$ The statistic {circumflex over (b)} can be used to determine the presence of intermodulation products by comparing the statistic {circumflex over (b)} to a predefined threshold.

In additional or alternative aspects, the intermodulation detection sub-system 103′ depicted in FIG. 7 can determine the distance from the isolation sub-system 714 and the signal strength for each intermodulation product. The statistic {circumflex over (b)} can be computed at different intermodulation frequencies f_(n). The intermodulation frequencies f_(n) can be represented as a function of the frequency indexes k₁ and k₂, as represented by the function

$f_{n} = {f_{tx} + {\frac{f_{s}}{L}\left( {{\frac{n + 1}{2}k_{1}} - {\frac{n - 1}{2}k_{2}}} \right)}}$ where the intermodulation frequency f_(n) is a linear function of k₁ and k₂. For the ψ₁ and ψ₂, represented by the equations

$\psi_{1,i} = {\frac{2\pi\; f_{s}\tau_{i}}{L}\left( {n + 1} \right)}$ and ${\psi_{2,i} = {\frac{2\pi\; f_{s}\tau_{i}}{L}\left( {n - 1} \right)}},$ the statistic {circumflex over (b)} can be represented by the equation

${\hat{b}\left\lbrack {k_{1},k_{2}} \right\rbrack} = {{\sum\limits_{i = 1}^{P}{b_{i}{\mathbb{e}}^{j{({{\psi_{1,j}k_{1}} + {\psi_{2,i}k_{2}} + {4\pi\; f_{tx}\tau_{i}} + \phi_{i}})}}}} + {d_{r}{{\mathbb{e}}^{{j\varphi}\; r}.}}}$

For one frequency index that is a constant and another frequency index that is varied, the test statistic {circumflex over (b)} can be a linear combination of complex sinusoids. Thus, the amplitudes b_(i) and frequencies ψ₁ and ψ₂ can be determined from the sequence {circumflex over (b)}[k₁,k₂]. The values can be determined using any suitable technique, such as (but not limited to) the Fourier technique or “super resolution” techniques such as MUSIC and ESPRIT. Time delays associated with the points of nonlinear discontinuity can be determined from one of the frequencies ψ₁ and ψ₂ depending on the frequency index varied to generate the test statistic sequence. Amplitudes can be estimated from the test statistic using the time delays.

Passive Detection of Intermodulation Products

In additional or alternative aspects, an intermodulation detection sub-system 103″ can be configured to perform passive detection of intermodulation products. FIG. 8 depicts an intermodulation detection sub-system 103″ configured to perform passive detection of intermodulation products. The intermodulation detection sub-system 103″ can include the processor 801, an intermodulation modeling module 802, a downlink capture module 804, and an uplink capture module 806.

The downlink capture module 804 can be communicatively coupled to the downlink path 104 via a directive coupler 808. The downlink capture module 804 can convert an analog downlink signal coupled from the downlink path 104 to a downlink digital data set representing a downlink signal.

The intermodulation modeling module 802 can apply a non-linear function to the downlink digital data set. Applying the non-linear function to the downlink digital data set can model the effect of the downlink signal generating an intermodulation product in the uplink path 106. The output of the intermodulation modeling module 802 can be a digital data set representing a model intermodulation product.

The uplink capture module 806 can be communicatively coupled to the uplink path 106 via the directive coupler 810. The uplink capture module 806 can convert an analog uplink signal coupled from the uplink path 106 to a digital uplink signal. The output of the uplink capture module 806 can be an uplink digital data set representing a digital uplink signal.

The uplink digital data set from the uplink capture module 806 and digital data set representing the model intermodulation product from the intermodulation modeling module 802 can be provided to the processor 801. The processor 801 can cross correlate the uplink digital data set and the digital data set representing the model intermodulation product. The processor 801 can determine that a mathematical correlation exists between the uplink digital data set and the model intermodulation product, thereby indicating the presence and signal power of an intermodulation product in the uplink path 106.

FIG. 9 schematically depicts an example of a downlink capture module 804 and an uplink capture module 806. The downlink capture module 804 can include the downlink capture buffer 901 and the down-conversion module 902 a. The down-conversion module 902 a can include a low noise amplifier 904 a, a filter 906 a, a mixer 908 a, a local oscillator 910 a, a filter 912 a, and an analog-to-digital converter 914 a. The uplink capture module 806 can include the uplink capture buffer 903 and the down-conversion module 902 b. The down-conversion module 902 b can include a low noise amplifier 904 b, a filter 906 b, a mixer 908 b, a local oscillator 910 b, a filter 912 b, and an analog-to-digital converter 914 a.

A downlink signal and an uplink signal can be separated and individually sampled via directional couplers 808, 810 coupled to the downlink path 104 and the uplink path 106, respectively. Each of the downlink signal and the uplink signal respectively coupled from the downlink path 104 and the uplink path 106 can be down-converted using mixers 908 a, 908 b and local oscillators 910 a, 910 b. The analog-to-digital converters 914 a, 914 b can synchronously sample the downlink signal and the uplink signal. The downlink capture buffer 901 can store the sampled downlink signal. The uplink capture buffer 903 can store the sampled uplink signal. In some aspects, the respective down-conversion modules 902 a, 902 b for the downlink signal and the uplink signal can be configured such that the entire downlink frequency band and uplink frequency band are captured. In other aspects, the down-conversion module 902 a for the downlink signal can be tuned to a portion of the downlink frequency band that includes signals having a signal power sufficient to produce intermodulation products in the uplink frequency band. The down-conversion module 902 b for the uplink signal can be tuned to a portion of the uplink frequency band in which intermodulation products can be present.

FIG. 10 depicts an example of the intermodulation modeling module 802 of an intermodulation detection sub-system 103″. The intermodulation modeling module 802 depicted in FIG. 10 can be communicatively coupled to and process data from the downlink capture buffer 901 and the uplink capture buffer 903 depicted in FIG. 9. The intermodulation modeling module 802 depicted in FIG. 10 can include a complex down-conversion module 1002, an interpolation module 1004, a non-linear function module 1006, a frequency-shifting module 1007, a decimator module, and a cross-correlation module.

The complex down-conversion module 1002 can complex down-convert the downlink signal and the uplink signal respectively received from the downlink capture buffer 901 and the uplink capture buffer 903. The complex down-conversion module 1002 can include mixers 1014 a, 1014 b, a local oscillator 1016, and filters 1018 a, 1018 b. Complex down-converting the downlink signal and the uplink signal can center the downlink signal and the uplink signal on zero and respectively convert the downlink signal and the uplink signal into a complex downlink signal and a complex uplink signal.

The interpolation module 1004 can interpolate the complex downlink signal by a factor of m such that a non-linear function applied by the non-linear function module 1006 does not cause aliasing.

The non-linear function module 1006 can apply a nonlinear function such as y[n]=|x[n]|^(m)x[n], which models the m^(th) order intermodulation that can occur as a result of nonlinear discontinuities in the telecommunications system 100.

The frequency-shifting module 1007 can shift the frequency of the complex downlink signal such that the intermodulation modeled by the nonlinear function is frequency aligned with intermodulation products generated by the telecommunications system 100 that are included in uplink signals traversing the uplink path 106. The frequency-shifting module 1007 can include a mixer 1008 and a local oscillator 1010.

The decimator module 1012 can decimate the complex downlink signal to an original sample rate. The processor 801 can cross correlate the complex downlink signal with the complex uplink signal. The presence of an intermodulation product in the complex uplink signal can cause the complex downlink signal to be correlated with the complex uplink signal, thereby causing the cross-correlation to have a significant peak. A maximum cross-correlation value can indicate that an intermodulation product is present and can indicate the signal power of the intermodulation product.

The foregoing description, including illustrated examples, of the invention has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of this invention. Aspects and features from each example disclosed can be combined with any other examples. 

What is claimed is:
 1. An intermodulation detection sub-system, comprising: a controller integrated into a unit of a telecommunications system, wherein the controller is configured to control a test signal generation module and at least one intermodulation detection device, and is further configured to select a first frequency of a first signal component of a test signal and a second frequency of a second signal component of the test signal, wherein the first frequency and the second frequency are selected such that the sum or difference of the first frequency and the second frequency is a third frequency of an intermodulation product in an uplink frequency band of the telecommunications system; the test signal generation module integrated into the unit, wherein the test signal generation module is configured to provide the test signal to a remote antenna unit via a downlink path of the telecommunications system, wherein the test signal generation module comprises: a first signal source configured to generate the first signal component of the test signal, a second signal source configured to generate the second signal component of the test signal, and a combiner configured to generate the test signal by combining the first signal component and the second signal component; the at least one intermodulation detection device integrated into the telecommunications system, wherein the at least one intermodulation detection device is configured to detect the intermodulation product; and a frequency scanner configured to detect an extraneous signal in a coverage area of the remote antenna unit, wherein the controller is further configured to select the first frequency and the second frequency such that the third frequency is different from a frequency of the extraneous signal; wherein the at least one intermodulation detection device is integrated into at least one of the remote antenna unit or the unit.
 2. The intermodulation detection sub-system of claim 1, further comprising at least one test switch, the at least one test switch configured to selectively couple the at least one intermodulation detection device to the downlink path of the telecommunications system or an uplink path of the telecommunications system, wherein the controller is further configured to generate a control signal for selectively coupling the at least one intermodulation detection device to the downlink path or the uplink path based on the intermodulation detection sub-system being in a test mode in which the test signal is provided to the downlink path.
 3. The intermodulation detection sub-system of claim 1, wherein the test signal generation module further comprises a test buffer communicatively coupled to the downlink path of the telecommunications system, the test signal generation module configured to generate the test signal using a test digital data set provided by the controller; wherein the at least one intermodulation detection device further comprises a capture buffer communicatively coupled to an uplink path of the telecommunications system, the capture buffer configured to generate an uplink digital data set from an uplink signal traversing the uplink path; wherein the controller is further configured to detect the intermodulation product in the uplink signal by correlating a model intermodulation product generated based on the test digital data set and the uplink digital data set from the capture buffer.
 4. The intermodulation detection sub-system of claim 3, wherein the test buffer and the capture buffer are synchronized to a common reference clock.
 5. The intermodulation detection sub-system of claim 1, wherein the at least one intermodulation detection device comprises a power measurement device integrated into the remote antenna unit, wherein the power measurement device is configured to measure the power of the intermodulation product.
 6. The intermodulation detection sub-system of claim 5, further comprising at least one path switch, the at least one path switch configured to selectively couple the test signal generation module to one of a plurality of downlink paths of the telecommunications system.
 7. The intermodulation detection sub-system of claim 5, wherein the power measurement device comprises: at least one filter between an antenna of the remote antenna unit and the power measurement device, wherein the at least one filter is configured to isolate the intermodulation product from the extraneous signal.
 8. The intermodulation detection sub-system of claim 5, further comprising at least one terminating switch having a first configuration and a second configuration, the at least one terminating switch configured to selectively couple, in the first configuration, a portion of the downlink path of the telecommunications system to a terminating load and to selectively couple, in the second configuration, the portion of the downlink path to an input from a base station in communication with the telecommunications system, wherein the controller is further configured to generate a control signal for setting the at least one terminating switch to the first configuration based on the intermodulation detection sub-system being in a test mode in which the test signal is provided to the downlink path.
 9. An intermodulation detection sub-system, comprising: a test signal generation module comprising a first buffer, the first buffer communicatively coupled to a downlink path of a telecommunications system, the test signal generation module configured to provide a test signal to the downlink path; a second buffer communicatively coupled to an uplink path of the telecommunications system, the second buffer configured to generate an uplink digital data set from an uplink signal traversing the uplink path; and a processor communicatively coupled to the first buffer and the second buffer, wherein the processor is configured to detect an intermodulation product in the uplink signal traversing the uplink path by correlating a model intermodulation product and the uplink digital data set from the second buffer, wherein the model intermodulation product is generated based on a test digital data set from the first buffer representing the test signal.
 10. The intermodulation detection sub-system of claim 9, wherein the test signal generation module is integrated into a unit of the telecommunications system and is configured to provide the test signal to a remote antenna unit of the telecommunications system.
 11. The intermodulation detection sub-system of claim 9, wherein the test signal generation module comprises at least one of a periodic playback buffer, a digital-to-analog converter, an up-conversion module, or a power amplifier.
 12. The intermodulation detection sub-system of claim 9, wherein the first buffer and the second buffer are synchronized such that the processor is configured to correlate data from the first buffer and data from the second buffer by correlating the model intermodulation product with the uplink digital data set.
 13. The intermodulation detection sub-system of claim 9, wherein the processor is further configured to determine a distance from a non-linear impedance source generating the intermodulation product.
 14. The intermodulation detection sub-system of claim 9, further comprising: a downlink capture module communicatively coupled to the downlink path, the downlink capture module configured to generate a downlink digital data set from a downlink signal traversing the downlink path; and an intermodulation modeling module configured to generate an additional intermodulation product from the downlink digital data set; wherein the processor is further configured to detect the additional intermodulation product by correlating the additional intermodulation product and the uplink digital data set.
 15. The intermodulation detection sub-system of claim 14, wherein the intermodulation modeling module is configured to apply a non-linear function to the data from the first buffer, the non-linear function modeling a modification to the test signal by a non-linear impedance source.
 16. A method for detecting intermodulation products in a telecommunications system, the method comprising: selecting, by a controller integrated into a unit of the telecommunications system, a first frequency of a first signal component of a test signal and a second frequency of a second signal component of the test signal, wherein the first frequency and the second frequency are selected such that the sum or difference of the first frequency and the second frequency is a third frequency of an intermodulation product in an uplink frequency band of the telecommunications system, wherein the intermodulation product is generated by a non-linear interface between components of the telecommunications system mixing the first signal component and the second signal component, wherein the controller the first frequency and the second frequency are selected such that the third frequency is different from a frequency of an extraneous signal detected in a coverage area of a remote antenna unit of the telecommunications system; providing, by a test signal generation module integrated in the unit, the test signal to the remote antenna unit via a downlink path of the telecommunications system, wherein providing the test signal comprises: generating the first signal component and the second signal component based on the controller selecting the first frequency and the second frequency, and generating the test signal by combining the first signal component and the second signal component; detecting, by at least one intermodulation detection device integrated into the telecommunications system, the intermodulation product.
 17. The method of claim 16, further comprising coupling a portion of the downlink path to the test signal generation module prior to providing the test signal to the remote antenna unit, wherein the portion of the downlink path comprises a signal path between the unit and the remote antenna unit.
 18. The method of claim 17, further comprising terminating an additional portion of the downlink path prior to a point at which the test signal is provided to the portion of the downlink path, wherein the additional portion of the downlink path comprises a signal path between a base station and the unit. 